Cochlear implant electrode array

ABSTRACT

A cochlear implant electrode assembly ( 10 ) comprising an elongate electrode carrier member ( 11 ), a bioresorbable stiffening element ( 15 ) and an outer layer ( 16 ) surrounding the stiffening element ( 15 ). The carrier member ( 11 ) is made of a resiliently flexible first material and has a plurality of electrodes ( 12 ) mounted thereon and has a first configuration selected to allow it to be inserted into an implantee&#39;s cochlea, and at least a second configuration wherein it is curved to match a surface of the cochlea. The bioresorbable stiffening element ( 15 ) has a configuration selected for biasing the elongate member ( 11 ) into the first configuration and is made of a second material relatively stiffer than the first material and which dissolves or softens on exposure to cochlear fluids to permit the elongate member ( 11 ) to at least approach or adopt the second configuration. The outer layer ( 16 ) surrounding the stiffening element ( 15 ) is made of a material sufficiently resiliently flexible to allow the elongate member ( 11 ) to at least approach or adopt the second configuration. The outer layer ( 16 ) has a first rate of cochlear fluid ingress therethrough and has at least one fluid ingress means ( 21 ) formed therein. The rate of cochlear fluid ingress through the fluid ingress means ( 21 ) is greater than the first rate of cochlear fluid ingress through the outer layer ( 16 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/149,642, now U.S. Pat. No. 7,272,449, filed Nov. 13, 2003, which wasa National Stage application of International ApplicationPCT/AU01/01231, filed Sep. 28, 2001, and which claims the benefit ofAustralian Patent Application PR 0542, filed Oct. 4, 2000, PR 0807,filed Oct. 17, 2000, PR 1005, filed Oct. 25, 2000 and PR 1778, filedNov. 29, 2000, all of which are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to an implantable device and, inparticular, to an implantable cochlear electrode incorporating abioresorbable stylet.

BACKGROUND OF THE INVENTION

Hearing loss, which may be due to many different causes, is generally oftwo types, conductive and sensorineural. Of these types, conductivehearing loss occurs where the normal mechanical pathways for sound toreach the hair cells in the cochlea are impeded, for example, by damageto the ossicles. Conductive hearing loss may often be helped by use ofconventional hearing aid systems, which amplify sound so that acousticinformation does reach the cochlea and the hair cells.

In many people who are profoundly deaf, however, the reason for deafnessis sensorineural hearing loss. This type of hearing loss is due to theabsence of, or destruction of, the hair cells in the cochlea whichtransduce acoustic signals into nerve impulses. These people are thusunable to derive suitable benefit from conventional hearing aid systems,because there is damage to or absence of the mechanism for nerveimpulses to be generated from sound in the normal manner.

It is for this purpose that cochlear implant systems have beendeveloped. Such systems bypass the hair cells in the cochlea anddirectly deliver electrical stimulation to the auditory nerve fibres,thereby allowing the brain to perceive a hearing sensation resemblingthe natural hearing sensation normally delivered to the auditory nerve.U.S. Pat. No. 4,532,930, the contents of which are incorporated hereinby reference, provides a description of one type of traditional cochlearimplant system.

Cochlear implant systems have typically consisted of two key components,namely an external component commonly referred to as a processor unit,and an implanted internal component commonly referred to as astimulator/receiver unit. Traditionally, both of these components havecooperated together to provide the sound sensation to an implantee.

The external component has traditionally consisted of a microphone fordetecting sounds, such as speech and environmental sounds, a speechprocessor that converts the detected sounds and particularly speech intoa coded signal, a power source such as a battery, and an externalantenna transmitter coil.

The coded signal output by the speech processor is transmittedtranscutaneously to the implanted stimulator/receiver unit situatedwithin a recess of the temporal bone of the implantee. Thistranscutaneous transmission occurs through use of an inductive couplingprovided between the external antenna transmitter coil which ispositioned to communicate with an implanted antenna receiver coilprovided with the stimulator/receiver unit. This communication servestwo essential purposes, firstly to transcutaneously transmit the codedsound signal and secondly to provide power to the implantedstimulator/receiver unit Conventionally, this link has been in the formof a radio frequency (RF) link, but other such links have been proposedand implemented with varying degrees of success.

The implanted stimulator/receiver unit typically included the antennareceiver coil that receives the coded signal and power from the externalprocessor component, and a stimulator that processes the coded signaland outputs a stimulation signal to an intracochlea electrode assemblywhich applies the electrical stimulation directly to the auditory nerveproducing a hearing sensation corresponding to the original detectedsound.

The external componentry of the cochlear implant has been traditionallycarried on the body of the implantee, such as in a pocket of theimplantee's clothing, a belt pouch or in a harness, while the microphonehas been mounted on a clip mounted behind the ear or on a clothing lapelof the implantee.

More recently, due in the main to improvements in technology, thephysical dimensions of the speech processor have been able to be reducedallowing for the external componentry to be housed in a small unitcapable of being worn behind the ear of the implantee. This unit hasallowed the microphone, power unit and the speech processor to be housedin a single unit capable of being discretely worn behind the ear, withthe external transmitter coil still positioned on the side of the user'shead to allow for the transmission of the coded sound signal from thespeech processor and power to the implanted stimulator unit.

With continuing future technological advancements it will be possible toprovide a cochlear implant system which is totally implanted within thehead of the user and requires no external devices to operate. Themicrophone will be implanted within the user as well as a power source,so that there will be no need to require an external link for the deviceto operate, at least for a period of time.

Together with improvements in available technology much research hasbeen undertaken in the area of understanding the way sound is naturallyprocessed by the human auditory system. With such an increasedunderstanding of how the cochlea naturally processes sounds of varyingfrequency and magnitude, there is a need to provide an improved cochlearimplant system that delivers electrical stimulation to the auditorynerve in a way that takes into account the natural characteristics ofthe cochlea.

It is known in the art that the cochlea is tonotopically mapped. Inother words, the cochlea can be partitioned into regions, with eachregion being responsive to signals in a particular frequency range. Thisproperty of the cochlea is exploited by providing the electrode assemblywith an array of electrodes, each electrode being arranged andconstructed to deliver a cochlea-stimulating signal within a preselectedfrequency range to the appropriate cochlea region. The electricalcurrents and electric fields from each electrode stimulate the ciliadisposed on the modiola of the cochlea. Several electrodes may be activesimultaneously.

It has been found that in order for these electrodes to be effective,the magnitude of the currents flowing from these electrodes and theintensity of the corresponding electric fields, are a function of thedistance between the electrodes and the modiola. If this distance isrelatively great, the threshold current magnitude must be larger than ifthe distance is relatively small. Moreover, the current from eachelectrode may flow in all directions, and the electrical fieldscorresponding to adjacent electrodes may overlap, thereby causingcross-electrode interference. In order to reduce the thresholdstimulation amplitude and to eliminate cross-electrode interference, itis advisable to keep the distance between the electrode array and themodiola as small as possible. This is best accomplished by providing theelectrode array in the shape which generally follows the shape of themodiola. Also, this way the delivery of the electrical stimulation tothe auditory nerve is most effective as the electrode contacts are asclose to the auditory nerves that are particularly responsive toselected pitches of sound waves.

In order to achieve this electrode array position close to the insidewall of the cochlea, the electrode needs to be designed in such a waythat it assumes this position upon or immediately following insertioninto the cochlea. This is a challenge, as the array needs to be shapedsuch that it assumes a curved shape to conform with the shape of themodiola and must also be shaped such that the insertion process causesminimal trauma to the sensitive structures of the cochlea. In this senseit has been found to be desirable for the electrode array be generallystraight during the insertion procedure.

Several procedures have been adopted to provide an electrode assemblythat is relatively straightforward to insert while adopting a curvedconfiguration following insertion in the cochlea. In one case, aplatinum wire stylet is used to hold a pre-curved electrode array in agenerally straight configuration up until insertion. Followinginsertion, the platinum stylet is withdrawn allowing the array to returnto its pre-curved configuration.

In another development, a bimetallic filament (such as nickel/titanium)or a shape memory alloy (such as an alloy of nickel and titanium) ispositioned in the electrode assembly and used to again hold a pre-curvedelectrode array in a generally straight configuration while the array isat about room temperature. On insertion into the body and exposure tobody temperature, the alloy or filament bends into a pre-selected curvedconfiguration.

In a still further arrangement, a longitudinal element that is arrangedon one side of the array and constructed to change its dimension oninsertion can be utilised. For example, the longitudinal element couldinclude a hydrogel, such as polyacrylic acid (PAA) or polyvinyl alcohol(PVA), which expands after insertion by absorbing water from thecochlear fluid.

In developing such electrode array designs, it is of great importancethat the design be constructed to minimise potential damage to sensitivestructures in the cochlear on insertion and placement. Each of the aboveconstructions suffer from a number of disadvantages in this regard.

Still further, it has been proposed to straighten pre-curved electrodearrays using inserted longitudinal elements or surrounding sheathsformed from bioresorbable materials that dissolve or soften onimplantation. A disadvantage with use of such bioresorbable materials isthat, due to the generally wet nature of the surgical environment, thepolymer can dissolve or soften before the electrode array isappropriately positioned, causing difficulties in placement andinsertion procedures.

The present invention is directed to an electrode assembly adapted toovercome some of the difficulties of prior art electrode assemblies.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

SUMMARY OF THE INVENTION

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

The present invention relates to an implantable tissue stimulatingdevice having a first configuration prior to and at least oncommencement of insertion into an implantee's body and adapted to adoptat least a second configuration following insertion.

According to one aspect, the present invention is an implantabletissue-stimulating device comprising:

an elongate member having a plurality of electrodes mounted thereon andhaving a first configuration selected to allow said member to beinserted into an implantee's body and at least a second configurationwherein said elongate member is adapted to apply a preselected tissuestimulation with the electrodes, said elongate member being made of aresiliently flexible first material;

a bioresorbable stiffening element having a configuration selected forbiasing said elongate member into said first configuration, saidstiffening element being made of a second material relatively stifferthan said first material and which dissolves or softens on exposure to afluid to permit said elongate member to at least approach or adopt saidsecond configuration; and

an outer layer surrounding the stiffening element, the layer being madeof a material sufficiently resiliently flexible to allow said elongatemember to at least approach or adopt said second configuration, theouter layer having a first rate of fluid ingress therethrough and havingat least one fluid ingress means formed therein, the rate of fluidingress through the fluid ingress means being greater than the firstrate of fluid ingress through the outer layer.

In a preferred embodiment, the second configuration of the elongatemember is curved. More preferably, the elongate member adopts a spiralconfiguration when in the second configuration.

In a further embodiment, the fluid is a saline solution. In anotherembodiment, the fluid is a body fluid of the implantee.

According to a second aspect, the present invention is a cochlearimplant electrode assembly comprising:

an elongate electrode carrier member having a plurality of electrodesmounted thereon and having a first configuration selected to allow saidmember to be inserted into an implantee's cochlea and at least a secondconfiguration wherein said elongate member is curved to match a surfaceof said cochlea, said elongate member being made of a resilientlyflexible first material;

a bioresorbable stiffening element having a configuration selected forbiasing said elongate member into said first configuration, saidstiffening element being made of a second material relatively stifferthan said first material and which dissolves or softens on exposure tocochlear fluids to permit said elongate member to at least approach oradopt said second configuration; and

an outer layer surrounding the stiffening element, the layer being madeof a material sufficiently resiliently flexible to allow said elongatemember to at least approach or adopt said second configuration, theouter layer having a first rate of cochlear fluid ingress therethroughand having at least one fluid ingress means formed therein, the rate ofcochlear fluid ingress through the fluid ingress means being greaterthan the first rate of cochlear fluid ingress through the outer layer.

The elongate member is preferably preformed from a plastics materialwith memory and is preformed to the second configuration. The elongatemember preferably has a first end that is firstly inserted into theimplantee.

In a preferred embodiment, the first configuration is preferablysubstantially straight. More preferably, the first configuration isstraight.

In a preferred embodiment, the elongate member is formed from a suitablebiocompatible material. In one embodiment, the material can be asilicone, such as Silastic MDX 4-4210. In another embodiment, theelongate member can be formed from a polyurethane.

In a further embodiment, the elongate member can have a resilientlyflexible tip member extending forwardly from the first end of the body.The tip member preferably has a distal end and a proximal end. The tipmember can have a stiffness that is relatively less stiff than saidstiffening element. The tip member can further be formed of a materialthat is substantially the same or the same stiffness as the body of theelongate member. In another embodiment, the tip member can be formed ofa material that is relatively less stiff than at least a portion of theelongate member. In a further embodiment, the tip member can be formedof a material that undergoes a change in stiffness, preferably adecrease in stiffness, on insertion into the body, such as the cochlea.

In a further embodiment, the stiffness of the tip member can vary alongat least a portion of its length from its distal end to its proximalend. In one embodiment, the stiffness of the tip member can vary overthe entire length of the tip member or only a portion thereof. Thestiffness can increase from the distal end to the proximal end. In oneembodiment, the stiffness of the tip member over said portion or itslength can increase gradually from its distal end towards to theproximal end. The increase in stiffness can be substantially smooth orincrease in a stepwise fashion.

In a further embodiment, the tip member can be formed of the samematerial as the body of the elongate member. In another embodiment, thetip member can be formed of a different material to that of the body ofthe elongate member. The tip member can be comprised of an innerrelatively stiff core of material having a tapered end, with at leastthe tapered end being overlaid by a relatively flexible material thatextends beyond the tapered end of the core material so that the tipmember undergoes a gradual decrease in flexibility in the region of thetapered end of the core moving away from the distal end.

The tip member can be formed separately to the body of the elongatemember and mounted thereto. For example, the tip member can be adheredto the first end of the body of the elongate member. In anotherembodiment, the tip member can be integrally formed with the body of theelongate member. The tip member can be formed from a silicone material.In another embodiment, the tip member can be formed of an elastomericmaterial, such as polyurethane.

In another embodiment, the tip member can have a plurality of metallicparticles dispersed therethrough. The metallic particles can besubstantially evenly dispersed through the tip member. Alternatively,the metallic particles can be non-evenly dispersed throughout the tipmember. In one embodiment, the metallic particles can increase indensity away from the distal end towards the proximal end of the tipmember. By varying the density of the metallic particles, it is possibleto vary the relative stiffness of the tip member.

The metallic particles preferably comprise a biocompatible material,such as platinum. The particles can be substantially spherical orspherical. It will be appreciated that the particles can have othersuitable shapes. In one embodiment, the particles can have a diameterbetween about 50 μm and 100 μm.

In addition to, or instead of, being used to potentially modify thephysical characteristics of the tip member, the provision of themetallic particles also result in the tip member being detectable byfluoroscopy and X-ray techniques. This provides another means for thesurgeon to monitor the placement and position of the tip member duringor after insertion of the electrode array in the body, such as in thecochlea.

When the elongate member is in the first configuration, the tip memberis preferably substantially straight and, more preferably, straight.

In a further embodiment, the tip member can be coated with a lubriciousmaterial. The lubricious material can be a bioresorbable ornon-bioresorbable material.

The tip member can be formed from, or incorporate as a portion thereof,a bioresorbable material. The presence of the bioresorbable materialpreferably results in the flexibility of the tip member increasing oninsertion of the tip member into the body, such as the cochlea. Thebioresorbable material in the tip member can be selected from the groupconsisting of polyacrylic acid (PAA), polyvinyl alcohol (PVA),polylactic acid (PLA) and polyglycolic acid (PGA).

In another embodiment, the tip member can be formed from, or incorporateas a portion thereof, a polymeric coating which becomes softer, and soincreases in resilient flexibility, in the presence of moisture or bodyheat.

The tip member preferably has a length from its distal end to itsproximal end in the range of about 0.3 to 4 mm, more preferably about1.0 to 3 mm. The diameter of the tip member can be substantiallyconstant for a majority of its length or can vary in diameter. The tipmember can be substantially cylindrical, cylindrical, or non-cylindricalfor a majority of its length. At the distal end, the diameter preferablygradually decreases to form a rounded end. The maximum diameter of thetip member is preferably about 0.55 mm.

In one embodiment, the tip member can be solid. In another embodiment,the tip member can have an external wall defining a cavity. In oneembodiment, the cavity can have a diameter greater than that of thereceiving portion of the body of the elongate member. In a furtherembodiment, the cavity can extend from the proximal end towards thedistal end of the tip member. The cavity can decrease in diameter awayfrom the proximal end. The cavity can be in communication with a distalend of the receiving portion of the body of the elongate member. In afurther embodiment, the stiffening means can extend into the cavity whenpositioned within the device or assembly according to the respectiveaspects of the present invention. In a preferred embodiment, the tipmember can move relative to the stiffening means when it extends intothe cavity of the tip member.

In general, the tip could be made of a combination of materials arrangedin a variety of geometries depending on the specific design goal. Theoutside shape and size of the tip can also be made in a variety of formsdepending on the design goal.

In a further embodiment, the bioresorbable material of the stiffeningelement is selected from the group consisting of polyacrylic acid (PAA),polyvinyl alcohol (PVA), polylactic acid (PLA) and polyglycolic acid(PGA). Other materials could also be used which provide thecharacteristics required for the particular application.

The outer layer can be in turn surrounded by another layer. In a furtherembodiment, the outer layer can be formed from a biocompatible material.In one embodiment, the outer layer can be formed from a material thathas a relatively higher degree of resilient flexibility than theelongate member. In another embodiment, the material of the outer layercan have a resilient flexibility identical to that of the elongatemember. In a preferred embodiment, the outer layer is formed from thesame material as the elongate member.

The outer layer can be bonded to the elongate member. In anotherembodiment, the outer layer can be integrally formed therewith.

In one embodiment, the stiffening element can comprise a longitudinalquasi-stylet disposed in a lumen extending within the elongate member.In one embodiment, the lumen can be cylindrical or any other suitableshape and also can have an opening formed therein, providing the fluidingress means of the assembly.

The opening is preferably at an end of the lumen distal the first end ofthe elongate member. In this embodiment, the opening can be closed by aclosure means adapted to seal the opening of the lumen.

The closure means can comprise a plug adapted to be inserted into thelumen and to form a seal therewith. The plug can have a frusto-conicalouter wall adapted to seal with the wall of the lumen on insertion. Theplug in this embodiment can be formed from a resiliently flexiblematerial such as silicone or polyurethane. Alternatively, the plug couldbe of any suitable shape and could also be formed from a stiff plasticsuch as polytetrafluoroethylene (PTFE) or a metal such as platinum orstainless steel.

In another embodiment, the closure means can comprise a cap adapted toseal the opening of the lumen. In one embodiment, the cap can have a topadapted to seal the opening and a skirt depending therefrom. The skirtcan have an engagement means formed on an outer surface thereof adaptedto engage with the inner surface of the lumen on mounting of the cap tothe lumen. In this embodiment, the inner surface of the lumen can havean engagement means complementary to that on the outer surface of theskirt of the cap. The engagement means on the skirt can comprise a screwthread adapted to engage with a corresponding screw thread on the innercylindrical surface of the lumen.

In another embodiment, the stiffening element can comprise a sheath thatat least partially surrounds the elongate member. In this embodiment,the sheath preferably fully envelops the elongate member. In thisembodiment, an annular channel can be formed in the elongate member toreceive the sheath. Such an annular channel can have an annular openingat an end distal the first end of the elongate member. The annularopening of the channel can be closed by a suitably shaped plug or cap.

In another embodiment, the opening of the lumen or channel can be closedby a sealing layer bonded to the elongate member. The sealing layer canbe formed from a layer of silicone material that is used to close theopening following placement of the stiffening element within theelongate member. In another embodiment, the elongate member can befabricated such that the closure means is provided by an extension ofthe outer layer over the opening of the lumen or channel or a drop ofsilicone or other sealing material over the opening.

In the latter case, the closure means is preferably removable to form anopening by slicing the sealing layer or outer layer extension with ablade, such as that provided by a pair of scissors, to allow ingress offluid into the lumen or channel.

In a further embodiment, a plurality of openings, comprising the fluidingress means, can be formed in the outer layer. In one embodiment, theopenings can be disposed along the length of the outer layer. In oneembodiment, the openings can be equally spaced along the length of theouter layer. In a preferred embodiment, the openings can comprise slitsformed in the outer layer. The slits preferably slow but do not preventingress of fluid through the outer layer to the stiffening element. In afurther embodiment, the slits can be formed in a lateral surface of theelongate member such that on the commencement of curvature of theelongate member, the slits are caused to at least partially open andallow fluid ingress into the elongate member.

The slits can be formed to all allow substantially the same or the samerate of ingress of fluid through the outer layer. In another embodiment,at least one slit can allow a different rate of progress of fluidthrough the outer layer compared to the other slits. In a still furtherembodiment, each slit can allow a different rate of progress of fluidthrough the outer layer compared to the other slits formed therein.

In one embodiment, the slit most distal the first end of the elongatemember can allow a greater rate of fluid ingress through the outer layerthan its adjacent slit positioned closer to said first end or viceversa. As such, the bioresorbable material beneath this slit preferablybegins to dissolve or soften before the remainder of the stiffeningelement so allowing the elongate member to begin to firstly move fromits first configuration to its second configuration at or adjacent theposition of this most distal or most proximal slit.

The rate of progress provided by each slit can follow this pattern alongthe length of the device towards the first end, with the next closerslit to the first end providing a relatively lesser rate of ingress thanits adjacent more distal slit. This pattern results in the bioresorbablematerial dissolving or softening from an end distal the first endtowards an end closer to the first end or vice versa. Where the firstconfiguration is straight and the second configuration is curved, theelongate member begins to curve distal the first end and then continuesto further adopt the curved configuration as the stiffening elementdissolves or softens towards the first end or vice versa.

In one embodiment, the slits or other fluid ingress means can besealable with a bioresorbable material. The bioresorbable materialpreferably softens and/or dissolves on exposure to a fluid, such ascochlear fluid, to allow ingress of the fluid into the elongate member.In this embodiment, the slits or other fluid ingress means can be sealedwith the same or a different quantity and/or the same or differentthicknesses of bioresorbable material. Variations in thickness and/orquantity of the bioresorbable material provide a means of varying therate of ultimate dissolution of the stiffening element of the device.

In another embodiment, the fluid ingress means can comprise regions ofsilicone having a thickness less than that of the remainder of the outerlayer. Due to the reduced thickness of these regions, the fluid passesthrough the fluid ingress means more quickly than the remainder of theouter layer. The thickness of the fluid ingress regions can be varied tosuit the desired rate of dissolution/softening of the stiffening elementrequired by the application. Different regions can have differentthickness as required. For example, a region distal the first end of theelongate member can have a thickness that is thinner than that of aregion closer to the first end.

In a further embodiment, at least a portion of an outer surface of theelongate member can have a coating of a lubricious material. In oneembodiment, a substantial portion or the entire outer surface of theelongate member can have a coating of the lubricious material.

In this embodiment, the lubricious material can be selected from thegroup comprising polyacrylic acid (PAA), polyvinyl alcohol (PVA),polylactic acid (PLA) and polyglycolic acid (PGA). It is envisaged thatother similar materials could also be used.

In a further aspect, the present invention comprises a method ofimplanting a tissue-stimulating device or cochlear electrode assemblydevice as defined herein in a body of an implantee.

In this aspect, the method can comprise a step of accessing theimplantation site and then a step of inserting the device. Prior toinsertion, the device is preferably substantially straight or straight.On insertion, the device can adopt an intermediate configuration (asdefined herein). Either prior to full insertion or following fullinsertion, the device preferably adopts its second configuration.

Once implanted, the electrodes can receive stimulation signals from astimulator means. The stimulator means is preferably electricallyconnected to the elongate member by way of an electrical lead. The leadcan include the one or more wires extending from each electrode of thearray mounted on the elongate member.

In one embodiment, the lead can extend from the elongate member to thestimulator means or at least the housing thereof. In one embodiment, thelead is continuous with no electrical connectors, at least external thehousing of the stimulator means, required to connect the wires extendingfrom the electrodes to the stimulator means. One advantage of thisarrangement is that there is no requirement for the surgeon implantingthe device to make the necessary electrical connection between the wiresextending from the electrodes and the stimulator means.

The stimulator means is preferably positioned within a housing that isimplantable within the implantee. The housing for the stimulator meansis preferably implantable within the bony well in the bone behind theear posterior to the mastoid.

When implantable, the housing preferably contains, in addition to thestimulator means, a receiver means. The receiver means is preferablyadapted to receive signals from a controller means. The controller meansis, in use, preferably mounted external to the body of the implanteesuch that the signals are transmitted transcutaneously through theimplantee.

Signals can preferably travel from the controller means to the receivermeans and vice versa. The receiver means can include a receiver coiladapted to receive radio frequency (RF) signals from a correspondingtransmitter coil worn externally of the body. The radio frequencysignals can comprise frequency modulated (FM) signals. While describedas a receiver coil, the receiver coil can preferably transmit signals tothe transmitter coil which receives the signals.

The transmitter coil is preferably held in position adjacent theimplanted location of the receiver coil by way of respective attractivemagnets mounted centrally in, or at some other position relative to, thecoils.

The external controller can comprise a speech processor adapted toreceive signals output by a microphone. During use, the microphone ispreferably worn on the pinna of the implantee, however, other suitablelocations can be envisaged, such as a lapel of the implantee's clothing.The speech processor encodes the sound detected by the microphone into asequence of electrical stimuli following given algorithms, such asalgorithms already developed for cochlear implant systems. The encodedsequence is transferred to the implanted stimulator/receiver means usingthe transmitter and receiver coils. The implanted stimulator/receivermeans demodulates the FM signals and allocates the electrical pulses tothe appropriate attached electrode by an algorithm which is consistentwith the chosen speech coding strategy.

The external controller further comprises a power supply. The powersupply can comprise one or more rechargeable batteries. The transmitterand receiver coils are used to provide power via transcutaneousinduction to the implanted stimulator/receiver means and the electrodearray.

While the implant system can rely on external componentry, in anotherembodiment, the controller means, including the microphone, speechprocessor and power supply can also be implantable. In this embodiment,the controller means can be contained within a hermetically sealedhousing or the housing used for the stimulator means.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the invention are nowdescribed with reference to the accompanying drawings, in which:

FIG. 1 is a simplified cross-sectional view of one embodiment of anelectrode assembly according to the present invention;

FIG. 2 is a simplified cross-sectional view of another embodiment of anelectrode assembly according to the present invention;

FIG. 3 is a diagrammatic view of the assembly of FIG. 1 being insertedin to the scala tympani;

FIG. 4 is a diagrammatic view of the assembly of FIG. 1 deployed in thescala tympani;

FIG. 5 is a simplified cross-sectional view of another embodiment of anelectrode assembly according to the present invention; and

FIGS. 6 a-6 d depict alternative tip structures for the electrodeassembly depicted in FIG. 5.

PREFERRED MODE OF CAMMING OUT THE INVENTION

One embodiment of a cochlear implant electrode assembly is depictedgenerally as 10 in FIGS. 1, 3 and 4.

The depicted electrode assembly 10 preferably has an electrical leadextending back to a stimulator/receiver housing. In considering thisinvention, it is to be understood that each electrode may have one ormore wires (not depicted) electrically connected thereto and extendingfrom each respective electrode back through the lead to thestimulator/receiver. The use of a stimulator/receiver as describedherein is known in the art and the present invention can be used withany such stimulator/receiver as known in the art.

The assembly 10 comprises an elongate electrode carrier member 11 havinga plurality of electrodes 12 mounted thereon. For the purposes ofclarity, the electrodes 12 depicted in FIGS. 1, 2 and 5 are notnecessarily shown to scale. The depicted elongate member 11 is preformedfrom a resiliently flexible silicone with memory and is preformed to acurved configuration suitable for conforming with the inner wall of thescala tympani 30 of the cochlea as depicted in FIG. 4. The elongatemember 11 has a first end 13, distal the lead, that is firstly insertedinto the implantee on insertion of the assembly 10.

As depicted in FIG. 5, the elongate member 11 can have a tip member 29integrally formed with its first end 13. The tip 29 is formed from thesame silicone used to fabricate the elongate member 11 and, in thedepicted embodiment, the material of tip member 29 has a resilientflexibility equal to that of the material used for the carrier member11.

Possible alternative constructions for the tip member 29 are provided inFIGS. 6 a-6 d. As depicted in FIG. 6 a, the tip member 70 can be solidand formed of an inner core 71 of relatively stiff material 71 and anouter layer 72 of relatively flexible material. The core 71 can taper indiameter over region 73 towards the distal end 21. The taper 73 causesthe overall stiffness of the tip 70 to increase over the length of thetaper 73 away from the distal end 21. The outer layer 72 can be formedof the same material as the remainder of the body of the elongatecarrier member 11 or can be a different material.

As depicted in FIG. 6 b, the tip member 40 can comprise a solid massintegrally formed to the first end 13 of the elongate carrier 11.

Still further and as depicted in FIG. 6 c, the tip member 50 cancomprise a solid mass 51 that is formed separately from the carriermember 11 and subsequently adhered thereto.

As depicted in FIG. 6 d, the tip member 60 can comprise an elastomericsilicone material having a plurality of substantially spherical platinumparticles 61 dispersed therethrough. The particles 61 have a diameterbetween about 50 μm and 100 μm. It will be appreciated that theparticles 61 depicted in FIG. 6 d are not drawn to scale.

In FIG. 6 d, the particles 61 are depicted as substantially evenlydispersed through the tip member 60. In another embodiment, theparticles could be non-evenly dispersed through the tip member. Forexample, the particles could increase in density away from the distalend 21 towards the proximal end of the tip member 60. By varying thedensity of the platinum particles 61, it is possible to vary therelative stiffness of the tip member 60.

In addition to, or instead of, being used to potentially modify thephysical characteristics of the tip member, the provision of themetallic particles 61 also result in the tip member 60 being detectableby fluoroscopy and X-ray techniques. This provides another means for thesurgeon to either monitor the placement and position of the tip member80 during or after insertion of the electrode array 10 in an implantee'scochlea.

Disposed within a substantially cylindrical lumen 14 is a stylet-typeelement 15. This stylet-type element 15 differs from a conventionalstylet in that it is formed from a bioresorbable polyacrylic acid (PAA)that is adapted to dissolve or soften on exposure to fluids to permitthe elongate member 11 to take its preformed curved configuration. Itwill be appreciated that the stylet could be formed from other suitablebioresorbable materials. The stylet-type element 15 has a straightconfiguration and has a stiffness greater than that of the siliconemaking up the elongate member 11. Accordingly, the stylet-type element15, when in position biases the elongate member 11 into a straightconfiguration as depicted in FIGS. 1 and 3.

Overlaying the stylet-type element 15 is an integral outer layer 16 ofsilicone material that surrounds and protects the stylet-type element15. In particular, the outer layer 16 serves to protect the stylet-typeelement 15, at least for some time, from dissolution or softening due toexposure of the assembly to fluids, such as cochlear fluids, oninsertion in the scala tympani 30.

As depicted in FIG. 1, the lumen 14 has an opening 17 at an end 18distal the first end 13. In the embodiment depicted in FIG. 1, theopening 17 can be closed by a plug 19 that is adapted to seal theopening 17 of the lumen 14. While a frusto-conical plug is depicted inFIG. 1, other plug types can be envisaged. For example, and as isdepicted in FIG. 2, the opening 17 can be sealed with a quantity 9 ofsilicone.

As an alternative to the plug 19 depicted in FIG. 1, the opening 17 ofthe lumen 14 can be closed by a sealing layer bonded to the elongatemember 11. The sealing layer can be formed from a layer of siliconematerial that is used to close the opening 17 following placement of thestylet-type element 15 within the lumen 14. In another alternative, theelongate member can be fabricated such that the closure is provided byan extension of the outer layer 16 over the opening 17 of the lumen 14.

In the latter case, the closure is preferably removed to form theopening 17 by slicing the sealing layer or outer layer extension with ablade, such as that provided by a pair of scissors, to allow ingress offluid into the lumen 14.

An alternative embodiment of the electrode array is depicted generallyas 20 in FIG. 2. In this embodiment, a plurality of transverse slits 21are formed in the outer layer 16. The slits preferably slow but do notprevent ingress of fluid through the outer layer 16 to the stylet-typeelement 15.

In the depicted embodiment, each slit 21 is adapted to allow asubstantially equal rate of ingress of fluid into the lumen 14. It willbe appreciated that the slit design could be modified such thatdifferent slits 21 allowed different rates of progress of fluid throughthe outer layer 16. For example, a slit 21 most distal the first end 13could be adapted to allow a greater rate of fluid ingress through theouter layer 16 than its adjacent slit positioned closer to the first end13. In this case, the bioresorbable material of the stylet-type element15 beneath this slit would begin to dissolve or soften before theremainder of the stylet-type element 15 so allowing the elongate member11 to begin to move from the straight configuration to its curvedconfiguration at or adjacent the position of this most distal slit 21.In the depicted embodiment, each slit 21 can also be filled with aquantity of bioresorbable material. In this case, each slit 21 can befilled with a different quantity or thickness of bioresorbable materialso as to provide a means of controlling the location of and rate ofdissolution of the stylet-type element 15.

The rate of progress provided by each slit 21 can follow this patternalong the length of the elongate member 11 towards the first end 13,with the next closer slit 21 to the first end 13 providing a relativelylesser rate of ingress than its adjacent more distal slit 21. Thispattern results in the bioresorbable material of the stylet-type element15 dissolving or softening from an end distal the first end 13 towardsan end closer to the first end 13. As such, the straight elongate member11 begins to curve distal the first end 13 and then continues to furtheradopt the curved configuration as the stylet-type element 15 dissolvesor softens towards the first end 13 or vice versa.

As an alternative to the slits 21, the outer layer 16 of the elongatemember 11 can be provided with one or more regions that more readilyallow ingress of bodily fluids, such as cochlear fluids. These regionscan comprise regions of the outer layer 16 that have a thickness lessthan that of the remainder of the outer layer 16. Due to the reducedthickness of these regions, the fluid passes through the regions morequickly than the remainder of the outer layer 16. The thickness of thefluid ingress regions can be varied to suit the desired rate ofdissolution of the stylet-type element 15 required by the application.Different regions can have different thickness as required. For example,a region distal the first end 13 of the elongate member 11 can have athickness that is thinner than that of a region closer to the first end13, or vice versa. Such regions may also be formed by a matrix ofpinholes or other such structure to allow for a chosen rate of fluidingress rather than slits.

In use, the substantially straight assembly 10 or 20 will initially bepositioned at an entry to the scala tympani 30 as depicted in FIG. 3. Atthis point, in those embodiments where present, the plug 19, or quantity9, can be removed or a covering over the slits 21 peeled away to allowbodily fluids, such as cochlear fluids, to move into the lumen 14. Entryof the fluids into the lumen 14 commences dissolution or softening ofthe stylet-type element 15.

As dissolution or softening is occurring, the assembly 10 can becarefully advanced into the scala tympani 30. Dissolution or softeningof the stylet-type element 15 causes the assembly 10 to begin to adopt acurved configuration. As the assembly 10 continues to be advanced, it ispreferably positioned as depicted in FIG. 4, with the electrodes 12facing the modiola within the cochlea so that they are positioned asclose as possible to the spiral ganglia thereof.

The control of the commencement of, and preferably the rate of, styletdissolution provides the surgeon with greater control of theimplantation procedure for the cochlear implant electrode assembly 10.The provision of greater control minimises the potential for trauma tothe sensitive tissues inside the cochlea and also enhances thelikelihood of successful placement of the assembly 10 at the firstattempt.

While the preferred embodiment of the invention has been described inconjunction with a cochlear implant, it is to be understood that thepresent invention has wider application to other implantable electrodes,such as electrodes used with pacemakers.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. An implantable tissue-stimulating device comprising: an elongatecarrier including a carrier body, the elongate carrier having aplurality of electrodes mounted thereon and having a first configurationselected to allow said member to be inserted into an implantee's bodyand at least a second configuration wherein said elongate member isadapted to apply a preselected tissue stimulation with the electrodes,said elongate carrier being made of a resiliently flexible firstmaterial; and a bioresorbable stiffening element having a configurationselected for biassing said elongate member into said firstconfiguration, said stiffening element being made of a second materialrelatively stiffer than said first material and which dissolves orsoftens on exposure to a fluid to permit said elongate carrier to atleast approach or adopt said second configuration, wherein thestiffening element extends in an interior of the carrier body along adirection of extension of the carrier body, the carrier body having atleast one fluid ingress formed therein placing the stiffening element influid communication with an exterior of the elongate carrier.
 2. Animplantable tissue-stimulating device of claim 1 wherein the secondconfiguration of the elongate carrier is curved.
 3. An implantabletissue-stimulating device of claim 2 wherein the elongate carrier adoptsa spiral configuration when in the second configuration.
 4. A cochlearimplant electrode assembly comprising: an elongate electrode carriermember having a plurality of electrodes mounted thereon and having afirst configuration selected to allow said member to be inserted into animplantee's cochlea and at least a second configuration wherein saidelongate member is curved to match a surface of said cochlea, saidelongate member being made of a resiliently flexible first material; abioresorbable stiffening element having a configuration selected forbiassing said elongate member into said first configuration, saidstiffening element being made of a second material relatively stifferthan said first material and which dissolves or softens on exposure tocochlear fluids to permit said elongate member to at least approach oradopt said second configuration; and an outer layer surrounding thestiffening element, the layer being made of a material sufficientlyresiliently flexible to allow said elongate member to at least approachor adopt said second configuration, the outer layer having a first rateof cochlear fluid ingress therethrough and having at least one fluidingress means formed therein, the rate of cochlear fluid ingress throughthe fluid ingress means being greater than the first rate of cochlearfluid ingress through the outer layer.
 5. A cochlear implant electrodeassembly of claim 4 wherein the stiffening element comprises a sheaththat at least partially surrounds the elongate member.
 6. A cochlearimplant electrode assembly of claim 5 wherein an annular channel isformed in the elongate member to receive the sheath.
 7. A cochlearimplant electrode assembly of claim 4 wherein a plurality of the fluidingress means in the form of slits are provided in the outer layer.
 8. Acochlear implant electrode assembly comprising: an elongate carrierincluding a carrier body, wherein the elongate carrier includes aplurality of electrodes mounted thereon; and a bioresorbable stiffeningelement extending in an interior of the assembly along a direction ofextension of the elongate carrier, wherein the assembly is configured topermit ingress of body fluid into the interior of the assembly throughthe carrier body so that the body fluid comes into contact with thestiffening element, wherein the stiffening element is configured to atleast one of dissolve or soften when in contact with the body fluid sothat resistance to flexure of the elongate carrier from a firstconfiguration that is substantially straight to a second configurationthat is substantially curved is reduced.
 9. A cochlear implant electrodeassembly of claim 8 wherein the elongate carrier is preformed from aplastics material with memory and is preformed in the secondconfiguration.
 10. A cochlear implant electrode assembly of claim 8wherein the elongate carrier is formed from a biocompatible material.11. A cochlear implant electrode assembly of claim 8 wherein thematerial of the stiffening element is selected from the group comprisingpolyacrylic acid (PAA), polyvinyl alcohol (PVA), polylactic acid (PLA)and polyglycolic acid (PGA).
 12. A cochlear implant electrode assemblyof claim 8 wherein the exterior surface of the elongate carrier issurrounded by a layer formed from a material selected from the groupcomprising a lubricious material and a non-lubricious material.
 13. Acochlear implant electrode assembly of claim 8 wherein the stiffeningelement is positionable in a lumen extending within the elongate member.14. A cochlear implant electrode assembly of claim 13 wherein the lumenhas at least one opening at an end of the lumen distal a first end ofthe elongate member, the opening being closable by a closure means. 15.A cochlear implant electrode assembly of claim 14 wherein the closuremeans comprises a plug or cap.
 16. A cochlear implant electrode assemblyof claim 14 wherein the opening is closable by a sealing layer bonded tothe elongate member.
 17. A cochlear implant electrode assembly accordingto claim 8, wherein the assembly includes one or more slits through thecarrier body that place the stiffening element in fluid communicationwith an exterior of the elongate carrier, thereby permitting the ingressof body fluid into the interior of the assembly.
 18. A cochlear implantelectrode assembly according to claim 8, wherein the assembly includes aplurality of openings through the carrier body that place the stiffeningelement in fluid communication with an exterior of the elongate carrier,thereby permitting the ingress of body fluid into the interior of theassembly, and wherein a rate of fluid ingress through a first opening ofthe plurality of openings is substantially greater than the rate offluid ingress through a second opening of the plurality of slits.
 19. Acochlear implant electrode assembly according to claim 18, wherein thefirst opening is located closer to the distal end of the carrier bodythan the second opening.